This invention relates to a method of treating an integrated circuit to provide a temperature sensor that is integral therewith.
It is conventional to operate a charge-coupled device (CCD) at a cryogenic temperature, in order to reduce dark current. When a CCD is operated at a cryogenic temperature, it is frequently desirable to monitor the temperature of the CCD die.
It is conventional to use a bipolar transistor to monitor the temperature of a CCD that is attached to a mounting block. A hole is drilled in the mounting block and a discrete bipolar transistor in a TO5 (or smaller) package is fitted in the hole. For a given base-emitter current, the base-emitter voltage of the transistor is linearly related to temperature. By supplying a constant base-emitter current and monitoring the base-emitter voltage, information regarding the temperature of the CCD can be obtained. In this type of temperature sensor, a diode may be used in lieu of a bipolar transistor.
Use of a discrete bipolar transistor or diode to monitor the temperature of a CCD in the manner described above is subject to disadvantage, because the temperature sensed by the transistor or diode is the temperature of the mounting block, not the temperature the CCD die.
The temperature of a CCD die may be monitored by use of a polysilicon resistor formed on the die during fabrication of the CCD. The resistor is provided with bonding pads for connection to an external circuit that monitors the resistance of the resistor. A disadvantage of this technique is that temperature-sensing resistors are not generally as sensitive to temperature as bipolar transistors or diodes.